An excellent penalty! (4)
I believe the answer is:
fine
This clue is a double definition.
'an excellent' is the first definition.
(I've seen this before)
'penalty' is the second definition.
(a financial penalty)
(Other definitions for fine that I've seen before include "Very good; penalty" , "Penalty; exalted; brandy" , "Fair - penalty" , "Satisfactory" , "Dry and sunny" .)